Mean streaming and the onset of turbulence in the reciprocating flow in a double bifurcation airway model: Insights for high-frequency ventilation

This paper reports on a numerical study of the reciprocating flow through a double bifurcation. Its aim is to quantify the potential gas transport in flows similar to those in the human airway during high-frequency ventilation, a medical ventilation technique that uses fast, yet shallow, inflations to minimize pressure and volume variation in the lungs and therefore protect lungs from ventilator-induced lung injury. The shallow inflations mean that gas transport is not achieved by bulk advection and other mechanisms must be used. Here we focus on nonlinear mean streaming and turbulent diffusion. We show that both of these mechanisms are driven by the formation of Dean vortices due to centrifugal instability in the curved sections of the bifurcation. We report the impact of the upstream and downstream conditions on both of these during both inhalation and exhalation portions of the reciprocating flow; in particular, we show that the development of turbulence in a given airway vessel is influenced by downstream conditions and not simply by the local flow conditions. This result highlights the importance of using a geometry of appropriate complexity for the modeling of physiological flows, particularly if the results are to be parameterised for use in larger-scale but lower-order models.